A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in the stars' atmospheres. The brightness increase is across the spectrum, from X rays to radio waves. The first known flare stars (V1396 Cygni and AT Microscopii) were discovered in 1924. However, the best-known flare star is UV Ceti, discovered in 1948. Today similar flare stars are classified as UV Ceti type variable stars (using the abbreviation UV) in variable star catalogs such as the General Catalogue of Variable Stars.
Most flare stars are dim red dwarfs, although recent research indicates that less massive brown dwarfs might also be capable of flaring. The more massive RS Canum Venaticorum variables (RS CVn) are also known to flare, but it is understood that these flares are induced by a companion star in a binary system which causes the magnetic field to become tangled. Additionally, nine stars similar to the Sun had also been seen to undergo flare events prior to the flood of superflare data from the Kepler observatory. It has been proposed that the mechanism for this is similar to that of the RS CVn variables in that the flares are being induced by a companion, namely an unseen Jupiter-like planet in a close orbit.
Nearby flare stars
Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth. On April 23, 2014, NASA's Swift satellite detected the strongest, hottest, and longest-lasting sequence of stellar flares ever seen from a nearby red dwarf. The initial blast from this record-setting series of explosions was as much as 10,000 times more powerful than the largest solar flare ever recorded.
The Sun's nearest stellar neighbor Proxima Centauri is a flare star that undergoes random increases in brightness because of magnetic activity. The star's magnetic field is created by convection throughout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun.
The flare star Wolf 359 is another near neighbor (2.39 ± 0.01 parsecs). Wolf 359, also known as Gliese 406 and CN Leo, is a red dwarf of spectral class M6.5 that emits X-rays. It is a UV Ceti flare star, and has a relatively high flare rate.
The mean magnetic field has a strength of about kG ( 2.2 T), but this varies significantly on time scales as short as six hours. 0.2  By comparison, the magnetic field of the Sun averages ( 1 G), although it can rise as high as 100 µT ( 3 kG) in active 0.3 Tsunspot regions.
A very low mass flare star is TVLM513-46546, slightly heavier than the lower limit for red dwarfs.
The most powerful stellar flare detected, as of December 2005, may have come from the active binary II Peg. Its observation by Swift suggested the presence of hard X-rays in the well-established Neupert effect as seen in solar flares.
- Schaefer, Bradley; King, Jeremy R.; Deliyannis, Constantine P. (February 2000). "Superflares on Ordinary Solar-Type Stars". The Astrophysical Journal (Astrophysical Journal) 529 (2): 1026. arXiv:astro-ph/9909188. Bibcode:2000ApJ...529.1026S. doi:10.1086/308325.
- Rubenstein, Eric; Schaefer, Bradley E. (February 2000). "Are Superflares on Solar Analogues Caused by Extrasolar Planets?". The Astrophysical Journal (Astrophysical Journal) 529 (2): 1031. arXiv:astro-ph/9909187. Bibcode:2000ApJ...529.1031R. doi:10.1086/308326.
- Kulkarni SR, Rau A (2006). "The Nature of the Deep Lens Survey Fast Transients". Ap J. 644 (1): L63. arXiv:astro-ph/0604343. Bibcode:2006ApJ...644L..63K. doi:10.1086/505423.
- NASA's Swift mission observes mega flares from nearby red dwarf star publisher on September 30, 2014 by ScienceDaily
- Christian DJ, Mathioudakis M, Bloomfield DS, Dupuis J, Keenan FP (2004). "A Detailed Study of Opacity in the Upper Atmosphere of Proxima Centauri". Ap J. 612 (2): 1140–6. Bibcode:2004ApJ...612.1140C. doi:10.1086/422803.
- Wood BE, Linsky JL, Müller HR, Zank GP (2001). "Observational Estimates for the Mass-Loss Rates of α Centauri and Proxima Centauri Using Hubble Space Telescope Lyα Spectra". Ap J. 547 (1): L49–L52. arXiv:astro-ph/0011153. Bibcode:2001ApJ...547L..49W. doi:10.1086/318888.
- Schmitt JHMM, Fleming TA, Giampapa MS (Sep 1995). "The X-Ray View of the Low-Mass Stars in the Solar Neighborhood". Ap J. 450 (9): 392–400. Bibcode:1995ApJ...450..392S. doi:10.1086/176149.
- Gershberg RE, Shakhovskaia NI (1983). "Characteristics of activity energetics of the UV Cet-type flare stars". Astrophys Space Sci. 95 (2): 235–53. Bibcode:1983Ap&SS..95..235G. doi:10.1007/BF00653631.
- Reiners A, Schmitt JHMM, Liefke C (2007). "Rapid magnetic flux variability on the flare star CN Leonis". Astronomy and Astrophysics 466 (2): L13–6. arXiv:astro-ph/0703172. Bibcode:2007A&A...466L..13R. doi:10.1051/0004-6361:20077095.
- Staff (January 7, 2007). "Calling Dr. Frankenstein! : Interactive Binaries Show Signs of Induced Hyperactivity". National Optical Astronomy Observatory. Retrieved 2006-05-24.
- "V2500 Oph". The International Variable Star Index. Retrieved 18 November 2015.
- UV Ceti and the flare stars, Autumn 2003 Variable Star Of The Season prepared by Dr. Matthew Templeton, AAVSO(www.aavso.org)
- Stellar Flares - D. Montes, UCM.